Abstract

Agarwood from Aquilaria plants, also known as Chen Xiang (沉香), is traditionally used for the treatment of abdominal pain and as a sedative. Because of the great demand and the rareness of agarwood, extensive harvesting of Aquilaria plants has nearly led to the extinction of the species. To fully utilize this resource, the use of different parts of Aquilaria needs to be investigated. This article will focus on the pharmacological properties and the mechanism of action of different parts of Aquilaria plants.

Introduction

Agarwood is a dark resinous material found in the trunk of wounded Aquilaria plants. It is also known as Chen Xiang (沉香) in Chinese, Kritsana in Thai, and Agar in India. The word Chen (沉) means sink, while Xiang (香) means fragrance or incense. As the Chinese name implies, this wood has a strong fragrance and is burnt as incense. The aromatic oils present in agarwood mean agarwood to be heavier than water; thus, agarwood sinks rather than floats when placed in water. Currently, there are 17 species of Aquilaria known to produce agarwood. Among these species, Aquilaria malaccensis and Aquilaria crassna are the most well-known species and are commonly found in the Southeast Asian countries such as Thailand, Indonesia, Cambodia, Laos, Vietnam, and Malaysia. Agarwood is formed unevenly in the stem of the plant. Naturally grown Aquilaria generally does not contain a large amount of agarwood, and it is estimated that only 10% of natural Aquilaria species are potent to produce agarwood.[¹] Generally, agarwood can only be obtained from injured Aquilaria plants. When injured, the plant responds to the stimulation by activating secondary biosynthetic pathways to produce a resinous compound. This response is initiated by injuries, such as wounds and bacterial[²] and fungal infections.[³] The main markets for agarwood are in the Middle East, South Asia, and East Asia. As the use of agarwood is increasing, there is a growing demand for agarwood. Because of both the difficulties in the production of agarwood and the high demand for the product, the price of agarwood ranges from 20-6000 USD/ kg per kilogram. In addition, the price of essential oils extracted from agarwood can be as high as 30, 000 USD/kg, depending on the grade and quality of the agarwood. The quality of agarwood was traditionally assessed by the resin content, density, color, scent/aroma, the agarwood-inducing method, formation time, and place of origin. Now, the quality assessment of agarwood is performed by chemical analysis which identifies the constituents in the agarwood.[⁴]

Agarwood is widely used in many areas of religion, literature, art, and medicine. In many religions, incense is burnt as a symbolic form of worship or offering to deities and spirits. This tradition can be traced back to ancient Egypt, where frankincense and myrrh resins were used to produce incense.[⁵] In China, the tradition of burning incense was widespread during the Song Dynasty. During that period, incense burning was considered one of the four arts of scholars, along with tea sipping, picture, and flower-branches arranging.[⁶] The ingredients for making incense mainly come from agarwood. Agarwood was a symbol of nobility. Incense can also be used for expelling insects that transmit diseases, such as mosquitoes and flies, which is a practice for disease prevention. The fragrance from incense can also be used for aromatherapy. The essential oil extracted from agarwood is added to cosmetics to enhance the scent of the products. The use of other parts of Aquilaria is often neglected when harvesting, as the wood from Aquilaria is soft and not suitable for making furniture. However, in some religions, Aquilaria wood is crafted into sculptures or ornaments.

Because of the great demand on agarwood, the extensive harvesting of Aquilaria has disrupted the natural growing cycle. In the past, agarwood was harvested from forests and the entire Aquilaria trees were cut down to obtain the dark resin inside the infected trunk. The resulting deforestation has led to the near extinction of the species. In 2005, Aquilaria species were listed as the endangered under the Convention on International Trade in Endangered Species Appendix II as endangered.[⁷] The governments of many countries are undertaking legal action to control the trade in Aquilaria species. More sustainable ways of producing agarwood have been investigated to increase the production rate and stop the destruction of natural forests. In addition, methods for agarwood induction have been investigated to increase the production of agarwood and prevent the extinction of Aquilaria. One of the methods used to harvest agarwood more sustainably is the infection and wounding of the trees. Farmers in Southeast Asia are starting to cultivate Aquilaria trees, and simple wounding techniques, such as cutting with an axe, nailing, or bark removal, are used to induce agarwood production. Because these techniques do not always produce the same amount of agarwood, therefore the production is not stable and can only produce a small amount of resin, and the products are considered to be low-grade agarwood.[⁸] Recently, scientific methods, such as fungal infection and chemical induction, have been investigated to increase the production of agarwood.[⁹] The following of this article will cover the medicinal use of agarwood in traditional Chinese medicine (TCM) and the pharmacological properties of agarwood.

In the TCM theory, agarwood has a spicy flavor, a warm property, and enters to the spleen, stomach, and kidney meridians. Agarwood was first described by Tao Hongjing (陶弘景) in the Ming Yi Bie Lu (《名医别录》 Miscellaneous Records of Famous Physicians) and has many different names, such as Xun Lu Xiang (薰陆香), Ji She Xiang (鸡舌香), and Feng Xiang (枫香).[¹⁰] Agarwood was described in the Ming Yi Bie Lu as being effective for the treatment of cholera and heart troubles. In a another book, the Hai Yao Ben Cao (《海药本草》 Overseas Materia Medica) described the use of agarwood for the treatment of psychological and neurological symptoms, such as mental problems and coma. The Overseas Materia Medica also mentioned the use of agarwood in ointments for treating swollen wounds.[¹¹] The Ri Hua Zi Ben Cao (《日华子本草》 Materia Medica of Ri Hua Zi) included the use of agarwood to treat cold and dampness related to pains in the joints.[¹²] The Yao Xing Fu (《药性赋》 Medicine Properties in Verse) added alleviating nausea and asthma to the uses of agarwood.[¹³] The Ben Cao Jing Shu (《本草经疏》 Commentary on the Materia Medica) described the effect of agarwood in regulating the ascending or descending of qi.[¹⁴] It also mentioned the property of agarwood to alleviate edema; the flavor of agarwood is spicy; thus, agarwood can dry dampness in the spleen and reduce the edema. The contraindications of agarwood are also mentioned in the Commentary on the Materia Medica. Agarwood is not suitable for patients with qi weakness of middle jiao affecting the flow of qi back to its origin and patients with excess pathogens in the heart meridian. The warming kidney property of agarwood is mentioned in the Yao Xing Jie (《药性解》 Exploration on Medicine Properties). Li Zhongzi (李中梓) observed that agarwood sinks when placed in water, and pointed out the properties of agarwood to direct qi downward and nourish yin.[¹⁵] Thus, agarwood is said to have an effect on the lower parts of the human body, such as the life gate (命门) and the kidneys. Because the use of aromatic and spicy herbs will lead to dryness, agarwood is suitable for slows down deficiency cold pattern but not in the case of excessive ministerial fire. The actions of agarwood are summarized in the Chinese Pharmacopoeia as moves qi and relieves pain, directs rebellious qi downward and stops vomiting, and warms kidneys to aid in qi absorption.[¹⁶] The following section will describe the use of agarwood in TCM and the pharmacological properties and mechanisms of action of the constituents of agarwood.